A magnetorheological fluid is a suspension of finely powdered iron or iron alloy in a fluid such as mineral oil or silicone. A magnetorheological fluid clutch may consist of this type of fluid suspension carried between clutch plates, with an associated device providing a desired magnetic flux level across the clutch plates and the fluid. The clutch plates are typically made of a material with high permeability such as iron. When the magnetic flux is generated across the clutch plates and through the magnetorheological fluid, the suspended particles respond. The response is embodied as an attraction of the clutch plates to the magnetorheological fluid particles. This characteristic phenomenon, combined with the internal magnetic attraction between the fluid particles, results in torque transmission between the clutch plates. In the past, magnetorheological fluid clutches have been referred to as magnetic particle fluid clutches. Many of the particle fluids used previously have been dry powders. With the development and use of suspensions of powders in a fluid medium such as mineral oil or silicone, studies were conducted into the rheology of particle fluids under a magnetic flux, and consequently the terminology of magnetorheological fluids has been coined.
Drives for the cooling fans of cars and trucks initially comprised a simple solid shaft extending from a drive pulley to the fan. Subsequently, the art realized that the fan did not require continuous engagement. It is known that the power consumed by driving a fan is proportional to the cube of the fan speed, whereas the cooling rate is typically proportional to the square root of the fan speed. Therefore, it is expected that considerable improvement in power consumption and fuel economy can be achieved by controlling fan activation so that the fan is disengaged when operating conditions permit. This realization led to the development of disengageable fan drives.
Typical production cars most commonly use electrically driven fans for readily disengaging the fan. The fan is driven independent of the engine through an electric motor, and the electric motor is turned on or off as needed. These electric motors are typically rated for a maximum of a few hundred watts. The power requirements of a cooling fan for trucks typically reach up to several kilowatts. Therefore, an electric fan drive is not practical for higher cooling requirement applications such as trucks.
The disengageable fan drive most commonly used in higher cooling rate applications, is the viscous fluid clutch. Torque transmission is typically achieved by the viscous drag force between two grooved clutch plates shearing a thin layer of silicone oil. The viscous fluid clutch is often composed of two sections, one contains the clutch grooves and performs the shearing function, and the other acts as a reservoir for the silicone fluid. Silicone fluid passes from the reservoir to the clutch grooves through an orifice. A thermostat valve situated on the clutch senses the temperature of the air leaving the radiator, and correspondingly opens or closes the orifice. This in turn, fills or drains the clutch grooves with the silicone fluid, engaging or disengaging the clutch.
While the viscous fluid clutch represents a significant improvement over the solid shaft drive, it continues to have certain drawbacks. The viscous clutch operates with bi-state capability meaning it can only be either completely engaged or completely disengaged. Engagement occurs at or near engine speed, even if the actual cooling requirement could be supplied at a lower speed. This results in inefficiencies by using more engine power for fan driving torque than may be required for the actual operating conditions encountered. The viscous fluid clutch also results in undesirable fan noise generation in many operating situations. The cyclic nature of thermostatic fan control causes the fan speed to oscillate, which can be particularly noticeable when the associated engine is at idle. The level of noise generation is typically proportional to the fifth or sixth power of the fan speed. Since the viscous fluid clutch is only bi-state, the fan speed is very high when the clutch is engaged resulting in increased noise generation. With a viscous fluid clutch, the temperature control system is also undesirably limited. A first limitation arises from the bi-state operational nature of the device, because the system provides only the two choices of maximum cooling or minimum cooling. A second limitation arises because the engine coolant temperature is indirectly sensed, from the air passing through the radiator. The resultant less than optimal temperature control, can lead to degraded engine performance and hunting of the fan clutch (resulting in more noise).
Providing adequate cooling is the key function of the radiator fan. The bi-state viscous fluid clutch only provides either maximum cooling or minimum cooling. Adding a means of modulating fan speed so that input torque is tailored to the amount of cooling required, avoids certain undesirable viscous fluid clutch characteristics. A modulating fan clutch controls the fan speed to achieve a constant cooling rate with the same total cooling as the cycling viscous fan clutch for every unit of time. Modulating control strategy reduces the maximum speeds that the fan operates at, potentially leading to improvements over the viscous fan clutch. For example, using existing relationships between fan speed, cooling rate, and fan power consumption, a modulating strategy can result in increases in engine operating efficiency. Reducing fan clutch driving load results in greater engine power availability. This becomes particularly significant in applications such as trucks, since the fan power consumption is of the order of several kilowatts at higher speeds, and is directly taken from the engine. Additionally, with reduced fan speeds, a significant reduction in fan noise is possible. Noise reduction is further achieved because of the elimination of the cycling of the fan clutch on and off.
Several types of fan drives to achieve fan speed modulation have been, and are being investigated by the automotive industry. While electric drives can be easily adapted to modulate the fan speed, they aren't practical in all applications. One known manner of achieving fan modulation uses a hydrostatic drive unit between the engine and the fan. In this type of device, the engine drives a pump, and the fluid from the pump drives a fluid motor with fluid flow control effected by valves. By controlling the fluid flow, the motor speed and the fan speed, are modulated. A hydrostatic drive offers the advantage of remote location of the fan with respect to the engine making it suitable for transverse engines. However, complexity and cost are concomitant drawbacks. The hydrostatic drives also typically suffer from undesirable inefficiencies, particularly when operated at partial loads.
Other types of modulating fan drives that have been investigated include the use of wet clutches that use controlled circulation of the silicone oil in the general viscous fluid clutch to modulate speed. Control difficulties and complexity are associated undesirable drawbacks with these methodologies. Packagability, simplicity, and the ability to smoothly control the torque capacity of a fan clutch continues to be an elusive combination of characteristics. Accordingly, the need for such a device continues to exist.